The invention relates generally to a method and apparatus for accurately positioning a focused energy beam, and in particular to a method and apparatus for positioning a focused laser beam, very precisely and at high speed, over a complex integrated circuit surface.
When integrated circuits are manufactured, many of the circuits are defective and until recently were disposed of as being uncorrectable. The ratio of the good circuits to the total number of circuits manufactured, often termed the yield of a manufacturing process, is very important to the profitability of a semiconductor manufacturing operation. The higher the yield, the greater the profitability of the operation. As circuits become more and more sophisticated, and correspondingly as the number of elements forming a semiconductor circuit increases, the likelihood of a circuit being defective increases. Consequently, the yield decreases and profitability decreases and/or the price of the more sophisticated semiconductor integrated circuits increases.
Often, it is the failure of only a few of the tens or hundreds of thousands of circuit elements, that is, diodes, transistors, etc., within the integrated circuit that cause the entire circuit to be thrown away. Recently, however, with respect to very regular integrated circuits such as memories, integrated circuit manufacturers have included spare circuit elements on the semiconductor chip. Thus, when an integrated circuit is tested prior to encapsulation, a defective element can be replaced by severing the connection thereto and connecting a spare in its place. This process, called memory repair, improves the yield of complex semiconductor circuits dramatically, for example by a factor of two or three.
The preferred method for severing connections in the integrated circuit is vaporization, that is, the fine conductors connecting the element to the rest of the circuit are vaporized using a focused laser beam. The conductors are usually several micrometers wide and fabricated of metal or polycrystalline silicon. The laser beam must be aimed wlth great precision at the semiconductor surface; and the focused spot must be very small, on the order of the width of the conductor being vaporized, so that adjacent conductors are not damaged.
At present, the positioning accuracy and spot size required by integrated circuit manufacturers strain the capability of the equipment that is commercially available. As integrated circuit feature dimensions grow smaller, the strain upon present equipment will become even greater. In addition, the integrated circuit overall dimensions are increasing, which, as described below, yet further strain the capability of present commercial equipment.
The commercially available equipment which can be employed for "repair" of the integrated circuit, to increase yield, falls into one of two general classes. In accordance with one class, the "X-Y beam positioner", a lens moves in an X-Y coordinate system over the surface of the integrated circuit. The lens must be positioned directly over the conductor to be severed during the vaporization process. A pair of mirrors are also provided. One mirror moves in one dimension (for example the X direction) only, and the other, fixed relative to the lens, moves in two dimensions (X and Y). Together, the mirrors direct a collimated beam from a fixed laser onto the movable lens. While this system provides high accuracy and small spot size, it is either relatively slow because of a high inertial weight required to minimize vibration of the moving parts; or the vibration resulting from a lower weight construction provides a limit on spot size which is insufficient for future integrated circuit repair applications (as discussed further below).
A second class of commercially available equipment, designated the "galvo beam positioner", consists of a fixed lens and a pair of rotating mirrors which change the direction of the collimated light incident thereon from a fixed laser. The laser beam source is directed by the mirrors through the fixed lens toward the semiconductor surface at a designated and changeable angle controlled by the angular positions of the mirrors. The lens transforms the changing direction of light entering its optics into a changing position of a focused beam on the integrated circuit surface. In this commercial equipment, the mirrors are rotated, and hence positioned, through angles determined by the limited angular field of the lens, by galvanometer motors, often referred to as "galvos". While the galvo beam positioner is much faster than the X-Y beam positioner because it has much less inertia, and is vibration free, it does not have the necessary accuracy, spot size, or field of view required by those more sophisticated circuit configurations which have a relatively large surface area. These downside factors exist first because the angular position transducers are not accurate enough when their moment of inertia, and therefore their diameter, is small, and second, because of the demand placed upon lens design and manufacturing for a lens having a high ratio of field size to spot size, for example on the order of 1,000 to 10,000 or more. Thus, in the galvo beam positioner, small spot size conflicts with the large field of view required to cover the entire, and presently growing, size of integrated circuits. (The conflict exists because the angular position errors can be reduced by reducing lens focal length but reduced focal length conflicts with the requirement of a large field of view.)
The X-Y positioner on the other hand, meets the needs of memory repair in that its field is large enough, and its spot size has adequate uniformity over the entire field. In addition, its accuracy is sufficient if the design is well engineered and the equipment is operated slowly enough to avoid positioning errors. It has however much more inertia than the galvo beam positioner and is therefore much slower in operation. Practically then, the throughput (repairs per hour) of the X-Y positioner is potentially much less than the galvo beam positioner. In order to minimize this disadvantage, the X-Y positioner is often designed to be lightweight. The lightweight design however reduces the positioner's rigidity and increases its vibration. Thus, while it becomes faster in settling within one limit of error (0.25 millimeters, for example), it becomes slower in settling within much lower limits, (for example, 0.001 millimeters), due to vibration. Thus at the lower limit, the vibration problem has the effect of limiting the effective positioning accuracy. Furthermore, vibration becomes even more of a limitation because no one has yet produced an X-Y beam positioner which is symmetric and dynamically balanced. On the other hand, it is relatively easier to mount mirrors to galvanometers so that the galvo beam positioner is dynamically balanced.
The X-Y beam positioner also experiences vibration and other repeatability errors in the direction parallel to the optical axis of the lens (i.e. normal to the surface of the integrated circuit). This affects the focus of the laser on the integrated circuit surface. Thus, as the spot size at the surface becomes smaller, for a given wavelength of light, the depth of field decreases and vibration in the direction of the optical axis begins to cause variations in spot size so that the focus, in effect, varies. This sets a limit to the spot size presently obtainable with commercial X-Y beam positioners because those units now in use must be lightweight to compete, commercially (and with respect to speed), with the galvanometer systems. It furthermore turns out that the spot size limitation of commercial X-Y positioners is very close to the limit experienced by the present galvanometer beam positioners. In the galvo systems, however, the causes are completely different and include variation of spot size over the field of view.
It is therefore a primary object of the invention to avoid the disadvantages of both the X-Y beam positioning system and the galvanometer beam positioning system while maintaining the advantages of both.
Other objects of the invention are providing a method and apparatus for positioning a focused laser beam on an integrated circuit with very high accuracy, small spot size which is uniform over the entire field of the integrated circuit, larger field size than is presently obtainable with commercial galvanometer beam positioners, high speed, minimum vibration, and high reliability.